CNC machines cut material from a workpiece to produce a specific part using a plurality of cutting tools in a sequence of machining operations. The CNC machine moves the tools and the workpiece in three-dimensional space in a pre-determined path, facilitating their interaction and resultant material removal. The examples of CNC machine tools shown in FIGS. 1a, 1b, 2 and 3 are more accurate and productive than manual machines.
A computer supervises the motion of the CNC machine by executing commands of a CNC program. These commands define the tool to be used, the tool's cutting feed and rotational speed information, and describe a tool path necessary to remove material for a given operation. The CNC machines recognize special languages that differ from one brand of machine to another. The languages of the CNC machines are typically cumbersome for humans and are not user friendly.
Computer Aided Manufacturing (CAM) software systems simplify the process of generating the CNC program in some cases. CAM systems accept graphic design input in an interactive manner, and often in a 3D solid model form. Traditional CAM systems grew out of Computer Aided Design (CAD) and Computer Aided Engineering (CAE) systems. The focus of CAD and CAE is on the functionality and properties of the entire part. In this context, solid modeling is essential to investigate various properties of the materials, e.g., such as heat distribution or deformation under stress.
In contrast to design and engineering applications, a machining application is essentially a surfacing process. Often only a small segment of the entire part surface is machined. As an example, only the mating surfaces of metal cast parts are machined, leaving most of the surface in rough form. The machinist is interested only in the surfaces on which he has to work and the relationship between these surfaces. Although the global coordinates of all the part elements are known in the solid model produced for the design and engineering application, for a machining operation the operator is required to create a local coordinate system for each surface that requires machining. This procedure is difficult and labor intensive for a machine tool operator.
A typical machine part includes of one or more surfaces wherein each surface may contain multiple standard mechanical features, e.g., holes, slots, pockets, grooves, or threads. A mechanical drawing of the part contains sufficient information about the geometrical shape, the features, and the material of the part to define the part. The features are typically referenced to a surface on which they are located. Each surface is drawn as a separate view of the part. The surfaces may be planar or curvilinear surfaces created by a milling procedure or a curvilinear surface or a surface of revolution created by a turning procedure. Machining the features involves using a large number of cutting tools and setting machining parameters for every operation. For example, a simple tapped hole may require a minimum four tools: center drill, drill, tap, and chamfering tool. Numerous parameters such as speed, feed rate, and depth of cut might be required for each operation. In order to optimize the manufacturing process, the sequence of machine operations may have to be changed from the order in which the data were initially entered. Cost effective production dictates minimizing time invested in generating a validated CNC program and maximizing tool and equipment utilization. Expert knowledge is essential in organizing an efficient production process.
Since the tools and sequence of operations for a CNC program in most CAM systems are a function of the machine's capabilities and the operator's practices (which differ from one machine shop to another), they are usually defined by the operators. Therefore, while traditional CAM systems provide assistance in tool path calculations for jobs with sculptured and free machine surfaces, they do not offer much help for machining parts with many distinct features that require using a large number of tools.
One way to simplify entry of tooling information is described in U.S. Pat. No. 4,521,860 to Kanematsu et al. This patent discloses a special CNC computer that allows entry of three modes of operation connected to specific keys on a front panel. Each mode contains a pre-determined number of operation units such as hole drilling and tapping, slot milling, or pocket milling. The units allow entry of a combination of geometrical and tooling information that can be reused. Establishing hard rules for entering machine data into predefined units allows the computer to reuse the information within the confines of one surface of a part.
In addition, there are CAM systems for programming planar (two-dimensional) milling machines that include automated CNC program generators for tooling intensive parts produced on CNC drilling, boring, or planar milling machines. These systems have the capability of subtly capturing the operator's expertise in machining part features on a flat surface and reapplying this knowledge to machining the same or similar features on the same or different parts. To further simplify the program preparation process, the system allows the operator to identify several planar surfaces on a part, program each surface separately, and then generate one CNC program for the entire part.
CNC Milling Machines and Machine Centers, shown for example in FIG. 1A, are used for producing essentially flat or sculptured surfaces and for machining features (such as holes, slots, and pockets) on these surfaces. In this system, the workpiece moves in a linear fashion, while the cutting tool simply rotates. Relative to the cutting tool, the part moves in the X-, Y-, and Z-directions. Multi-Axis Machining Centers, shown for example in FIG. 1b, allow tilt to A-, B-, C-angles around X-, Y-, and Z-axes, respectively. Simpler machines, like CNC drills and bores, cut material only on one axis.
A CNC lathe (CNC Turning Machine), shown for example in FIG. 2, produces parts having curvilinear inner and outer surfaces of revolution, e.g., solid or hollow cylinders, cones, semi-spheres, and parts with surface features created by rotational movement of the part including grooves and threads. To create a round surface, the CNC lathe rotates the workpiece while moving the cutters in the plane of the rotational axis. In this respect, the CNC lathe differs substantially from CNC mills, drills, and bores. The CNC lathe cutters typically have only one cutting surface. This places additional constraints on the tool orientation in the process of automatic CNC program generation.
The newest CNC Turn-Mill Machining Centers, shown for example in FIG. 3, combine the capabilities of CNC milling, drilling, boring, and turning machines. A complex part having planar and curvilinear surfaces can be completely manufactured by one machine in one set-up. The Turn-Mill machines make parts with greater precision than other CNC machines because they eliminate the need to reposition the workpiece between turning and milling operations. The programming of these powerful machines, however, represents a significant challenge for the machine operators.